In Canada, South America and China, there are very large oil sands deposits known as oil-rich bitumen sands. These oil sands consist of a mixture of crude bitumen (a semi-solid form of crude oil), silica sand, clay minerals, and water. The Canadian oil sand deposits cover over about 141,000 square kilometers and have about 28 billion cubic meters (174 billion barrels) of economically recoverable crude bitumen. There are two ways to recover the crude bitumen from these oil sand deposits. One way is mining and the other, is in-situ operations. In Canada, approximately 15.6 billion cubic meters (98 billion barrels) of crude bitumen deposit can be recovered via in-situ operations. Steam Assisted Gravity Drainage (SAGD) is the main in-situ operation to recover crude bitumen from these oil sand deposits. SAGD uses dual horizontal wells, situated one on top of another, to recover the crude bitumen from these oil sand deposits. The main challenge of drilling the horizontal wells through these oil sand deposits is the bitumen sticking to the drilling components. The mixture of the crude bitumen or heavy oil, often referred to as tar sands, makes the drilling cuttings from these formations malleable and sticky. Bitumen is a mixture of hydrocarbon compounds containing the other elements of nitrogen, sulfur, oxygen, metals and the like. Bitumen is known to have, overall, an anionic charge and hydrophobic surfaces. When the conventional water-base drilling fluids are used to drill through the oil sand deposits, the anionic polymer additives in the drilling fluids increase the anionic surface charge of bitumen particles, causing bitumen to disperse into the drilling fluids. The dispersed bitumen particles in the drilling fluids re-aggregate on the surfaces of metal or plastic or oil-like substances, which have very strong affinity for the bitumen or heavy oil. The re-aggregated bitumen sticks to oil sand cuttings and causes the bitumen accretion or sticking problem known to occur while drilling oil sands deposits. Bitumen accretion causes many drilling problems, such as high torque and drag, slow travel, blinding of the shakers while drilling oil sand deposits and getting the liners stuck while running in the liners.
Many attempts have been tried in the field operations to overcome the bitumen sticking problem. One solution has been the addition of solvents into the drilling fluid to dissolve the bitumen (see for example, Canadian patent application no. 2,481,543, published Mar. 14, 2006, and Canadian patent no. 2,454,312 published Jan. 30, 2003). Other solutions include the addition of a mixture of phosphonates and phosphate esters into the drilling fluid to prevent the bitumen from sticking to the drilling components (see U.S. Pat. No. 7,081,438, issued Jul. 25, 2006), the addition of cationic polymers into the drilling fluid to encapsulate the bitumen (Canadian patent no. 2,508,339, published Jun. 17, 2004), and the addition of the surfactants into the drilling fluid to emulsify the bitumen (Canadian patent no. 2,451,585, published Jun. 2, 2004).
There are some specific deficiencies in the use of the above-mentioned treatments in SAGD drilling. For example, field operations have proved that the technique of U.S. Pat. No. 7,081,438 does not satisfactorily prevent the bitumen from sticking while drilling through the oil sand deposits.
The addition of solvents that dissolve bitumen to the drilling fluids (as in CA 2,481,543 and CA 2,454,312, vide supra) not only dissolve the bitumen from the cuttings, but also dissolve the bitumen from the oil sand formations, which results in a problem known as “hole washing out”. This also makes the solvents in the drilling fluids become quickly saturated with bitumen and means that the drilling fluid must be refreshed from time to time once the solvents in the drilling fluid are saturated with the bitumen and can no longer dissolve the bitumen from the cuttings. Further, it is very difficult to separate the solvents/bitumen from the drilling fluids in field operations using the existing drilling apparatus.
Cationic polymers used for the encapsulation of bitumen in drilling fluids (as in CA 2,508,339, vide supra) are not compatible with the conventional anionic drilling fluid additives, such as, polyanionic cellulose (PAC) and xanthan gum, because of the cationic/anionic interaction of the polymers. This interaction makes it is very difficult to control the rheology and filtrate loss when this drilling fluid is used.
Though the emulsified polymer drilling fluid described in CA 2,451,585 (vide supra) has been successful in drilling Cold Lake Oil Sands deposits, it has not worked well in drilling Athabasca Oil Sands deposits because the bitumen of Athabasca Oil Sands is much more sticky and difficult to be emulsified compared to the bitumen of Cold Lake Oil Sands. Further, the surfactants of CA 2,451,585 present a toxic, environmental issue and a foaming problem.
Hydrophobically associating polymers have been used in some oilfield applications, for example, as viscosifiers in enhanced oil recovery (polymer flooding), in drilling/completion fluids, as acid stimulations and as drag reducing agent as described in Han et al. Soc. of Petroleum Engineers, 104432, pp. 1-6, 2006 and in Taylor K. C. et al. Canadian International Petroleum Conference, Jun. 12-14, 2007, and in oily water clean up as described in U.S. Pat. No. 4,734,205.